1. Field of the Invention
The present invention relates to a secondary air supply system for an internal combustion engine for supplying a secondary air to an exhaust pipe of the engine for the purpose of reducing emission of harmful components of exhaust gases.
2. Description of the Prior Art
In a conventional secondary air supply system, a relief-pressure control valve is provided in a secondary air supply line connecting an air pump with the exhaust pipe of the engine so as to maintain the pressure in the secondary air supply line below a predetermined value. An air switching valve is also provided in the secondary air supply line for cutting off the supply of the secondary air in accordance with operational conditions of the engine. In the conventional relief-pressure control valve, the relief pressure is designed at a relatively low value for elongating the lifetime of the air pump.
Generally speaking, the emissions of unburned hydrocarbon (HC) and carbon monoxide (CO) are extremely increased during a rapid acceleration of the engine. It is, therefore necessary to supply an increased amount of the secondary air to the exhaust pipe during the accelerations.
In the conventional secondary air supply system described above, however, since the air from the air pump is bypassed to the atmosphere when the pressure of the air in the secondary air supply line exceeds the predetermined relief-pressure, the sufficient amount of the secondary air may not supplied to the exhaust pipe even though the air pump can supply the increased amount of air therefrom.
According to the prior art of this kind, which is disclosed in a Japanese laid-open utility model application 50-52307, the secondary air control valve has a pressure chamber to which the intake vacuum is applied during a normal running of the engine, a deformable diaphragm and a valve head connected to the diaphragm for controlling the amount of the secondary air in accordance with the intake vacuum applied to the pressure chamber.
An acceleration detecting device is further provided for increasing the amount of the secondary air during the accelerations. The device supplies the air from the air pump to the pressure chamber of the secondary air supply valve during the acceleration so that almost all of the air from the air pump may be supplied to the exhaust pipe of the engine.
As above, the prior art can supply the increased amount of the secondary air to the exhaust pipe during the accelerations, however it is disadvantageous that the amount of the secondary air can not be controlled in accordance with the degrees of the accelerations, and that the excess amount of the secondary air may be supplied to the exhaust pipe which would cause the over-heat of a catalyst or the like disposed in the exhaust pipe.